Abstract:

An amine is prepared by cleaving a carbamate of the formula I
wherein R1=hydrogen, alkyl, aryl or phenyl group, R2=alkyl, aryl
or phenyl group, wherein R1 and R2 are independently
substituted or unsubstituted, wherein the cleaving of the carbamate is
performed in the presence of an acid of the formula II
wherein R3, R4 and R5 are independently alkyl or phenyl
group, wherein R1, R2, R3, R4 and R5 are the
same or different.

Claims:

1. A process for preparing an amine by cleaving a carbamate of the formula
IwhereinR1=hydrogen, alkyl, aryl or phenyl group,R2=alkyl, aryl
or phenyl group,wherein R1 and R2 are independently substituted
or unsubstituted,wherein the cleaving of the carbamate is performed in
the presence of an acid of the formula IIwherein R3, R4 and
R5 are independently alkyl or phenyl group,wherein R1, R2,
R3, R4 and R5 are the same or different.

2. The process as claimed in claim 1, wherein, in the acid of the formula
II, R4 and R5 are each a methyl group and R3 is a group of
the formula III--(CH2)n--CH3 IIIwherein n=0-8.

3. The process as claimed in claim 1, wherein the acid used is neodecanoic
acid.

4. The process as claimed in claim 1, wherein the carbamate is used as a
solution, suspension or as a solid which comprises a solvent.

5. The process as claimed in claim 4, wherein the solvent of said
solution, suspension or said solid comprising a solvent has a boiling
point of 30.degree. C. to 300.degree. C.

6. The process as claimed in claim 1, wherein the acid is initially
charged in a reaction zone at a desired reaction temperature and only
then is the carbamate supplied to the reaction zone.

7. The process as claimed in claim 1, wherein 0.1 to 1000 molar eq. of
acid is used, based on a total amount of the carbamate.

8. The process as claimed in claim 1, wherein the cleaving of the
carbamate is performed at a reaction temperature of 50.degree. C. to
320.degree. C.

9. The process as claimed in claim 1, wherein the carbamate to be cleaved
is used as a solution or as a suspension, wherein carbon dioxide and the
solvent of the carbamate are removed from a reaction zone without
removing the amine.

10. The process as claimed in claim 1, wherein said amine remains in the
acid which serves as a catalyst and as a solvent for the amine.

11. The process as claimed in claim 1, comprising separately removing
carbon dioxide formed and the amine.

12. The process as claimed in claim 1, wherein reformation of the
carbamate is substantially prevented.

13. The process as claimed in claim 1, wherein reformation of the
carbamate is prevented.

14. The process as claimed in claim 1, wherein the amine to be isolated
from a reaction zone does not react with the acid to form an acid amide.

15. The process as claimed in claim 1, wherein said amine is aliphatic or
aromatic.

16. The process as claimed in claim 1, which proceeds without the use of
metals and/or or metal compounds.

17. The process as claimed in claim 1, wherein a mixture of acids of
formula II is used.

18. The process as claimed in claim 1, wherein said acid has a boiling
point of at least 150.degree. C. at a pressure of 1013 mbar.

19. The process as claimed in claim 1, wherein, in said carbamate of the
formula I, R1=hydrogen and R2=alkyl group which is substituted.

20. The process as claimed in claim 1, wherein, in said carbamate of the
formula I, R1=hydrogen and R.sup.2.dbd.--(CH2)n--NH2
wherein n=1-8.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to a process for preparing amines by
catalytic cleaving of carbamates.

[0003]2. Discussion of the Background

[0004]The cleaving, i.e. the decarboxylation, of carbamates is a common
process for preparing amines. The resulting amines are in turn starting
compounds for numerous industrial intermediates and active pharmaceutical
ingredients.

[0005]The general reaction mechanism of the cleaving of carbamates is
described by Ewing et al. in their publication J. Amer. Chem. Soc. (1980)
102(9), 3072-3084:

[0006]The cleaving of carbamates in the presence of tetra-n-butylammonium
fluoride (Bu4NF) is described by Jacquemard et al. in Tetrahedron
(2004) 60(44), 10039-10047:

[0010]The general structure of the carbamates is described by Gaines in J.
Org. Chem. (1985) 50, 410-411 as a mono-molecular zwitterion or disalt.

[0011]Japanese publication JP 2006-069941 A describes, inter alia, the
thermal decomposition of carbamates to prepare isocyanates. In contrast,
JP 2004-262892 A describes the thermal decomposition of carbamates in the
presence of a tin catalyst and of a solid catalyst selected from
silicates, silica gel and/or metal.

[0012]The decomposition of ammonium carbamate to ammonia and carbon
dioxide is described by WO 2006/094541 A and EP 1 195 194 A.

[0013]The cleaving of carbamates in the presence of a tin catalyst is also
described in JP 2004-262892 A. The use of iodine to cleave carbamates is
described by Vatele et al. in Tetrahedron Letters (2003), 44(51),
9127-9129.

DETAILED DESCRIPTION OF THE INVENTION

[0014]It was an object of the present invention to provide a process for
cleaving carbamates, which allows the carbamate to be cleaved at low
temperatures with removal of the carbon dioxide. In particular,
reformation of the carbamate from the carbon dioxide and desired amine
reaction products should be avoided. In particular, it was an object of
the invention to develop a continuous preparation process for this
purpose.

[0015]This and other objects have been achieved by the present invention
the first embodiment of which includes a process for preparing an amine
by cleaving a carbamate of the formula I

[0019]wherein R1 and R2 are independently substituted or
unsubstituted,

[0020]wherein the cleaving of the carbamate is performed in the presence
of an acid of the formula II

[0021]wherein R3, R4 and R5 are independently alkyl or
phenyl group,

[0022]wherein R1, R2, R3, R4 and R5 are the same
or different.

[0023]It has been found that, surprisingly, a process for preparing amines
by cleaving carbamates in the presence of an acid enables a continuous
procedure. The acids used in the process according to the invention are
high-boiling neo acids, which surprisingly possess catalytic action in
the carbamate cleaving. In addition to the catalytic action, these acids
also serve as bottom diluents in the process according to the invention.

[0024]It was all the more surprising that it was possible to use the
carbamate to be cleaved as a solution or else as a suspension, while
being able to remove carbon dioxide and the solvent of the carbamate used
from the reaction zone without also removing the amine target product.
Owing to the lower reaction temperature which is enabled through the use
of the acids, the amine target product remains in the acid which thus
serves, as well as a catalyst, also as a solvent for the amine. In this
way, the process according to the invention enables the separate removal
of the carbon dioxide formed and of the amine target product. The
reformation of the carbamate can thus be prevented. This is a crucial
advantage over the conventional processes, since the cleaving of the
carbamate here proceeds at higher temperatures, such that both the carbon
dioxide and the amine target product are transferred to the gas phase and
thus reform the carbamate.

[0025]It was also not obvious that it would be possible to remove the
amine to be isolated from the reaction zone in a simple manner and that
it would not react with the acid to form an acid amide. Instead, the
amine can be isolated completely and in very high purity from the
reaction zone which contains the acid. The process according to the
invention can surprisingly be applied to a large number of carbamates,
such that it is possible to prepare both aliphatic and aromatic amines in
this way. In this way, it is possible to provide a semibatchwise process
and, in a specific embodiment, also a continuous process. The process
according to the invention also has the advantage that it proceeds
without the use of metals and/or or metal compounds.

[0026]The invention thus provides a process for preparing amines by
cleaving carbamates of the formula I

[0027]where R1=hydrogen, alkyl, aryl or phenyl group,
[0028]R2=alkyl, aryl or phenyl group, [0029]where the substituents
of the R1 and R2 type are substituted or unsubstituted,wherein

[0030]the cleaving of the carbamate is performed in the presence of an
acid of the formula II

[0031]where R3, R4 and R5=alkyl or phenyl group,

[0032]where the substituents of the R1, R2, R3, R4 and
R5 type are the same or different.

[0033]The acids used in the process according to the invention are
compounds of the formula II. In the process according to the invention,
preference is given to using so-called neo acids or Koch acids. These
acids have the advantage that they, owing to the quaternary a-carbon
atom, are particularly hydrolysis- and oxidation-stable. Preference is
given to using acids of the formula II which have an alkyl group as
substituents of the R3, R4 and R5 type, especially acids
of the formula II having a methyl group as the substituent of the R4
and R5 type and, as the substituent of the R3 type, a group of
the formula III

(CH2)n--CH3 III

[0034]where n=0-8, preferably n=2-7.

[0035]In the process according to the invention, particular preference is
given to using high-boiling neo acids, especially having a total number
of carbon or 5 to 12. Very particular preference is given to using
neodecanoic acid (R4 and R5=methyl,
R3═--(CH2)5--CH3) as the acid. In the process
according to the invention, it is also possible to use mixtures of these
acids of the formula II, in which case these mixtures of acids may also
comprise isomers of the acids of the formula II.

[0036]It is advantageous to use acids which have a boiling point of at
least 150° C. at a pressure of 1013 mbar. The acids used in the
process according to the invention preferably have a boiling point of 160
to 300° C. The boiling point includes all values and subvalues
therebetween, especially including 170, 180, 200, 210, 220, 230, 240,
250, 260, 270, 280 and 290° C.

[0037]The acids used in the process according to the invention are notable
for their high boiling point, but also for their acid-catalytic activity
in the cleaving of carbamates.

[0038]In the process according to the invention, carbamates of the formula
I are cleaved to the corresponding amines. Preference is given to using
carbamates of the formula I where R1=hydrogen and R2=alkyl
group, but particular preference is given to using carbamates of the
formula I where R1=hydrogen and
R2═--(CH2)n--CH3 where n=0-8, preferably n=2-7,
in the process according to the invention. In a particular embodiment of
the process according to the invention, carbamates of the formula I where
R1=hydrogen and R2=alkyl group, where the alkyl group is
substituted, are used. In particular, carbamates of the formula I where
R1=hydrogen and R2=alkyl group which is substituted by amino
groups, preferably one amino group, are suitable for this process.
Preference is thus given to using carbamates of the formula I where
R1=hydrogen and R2═--(CH2)n--NH2 where
n=1-8, especially 2-7.

[0039]In the process according to the invention, the carbamate to be
cleaved can be used as a solution, suspension or as a solvent-comprising
solid. In the process according to the invention, preference is given to
using solvents which have a boiling point of 30 to 300° C., more
preferably of 50 to 150° C. The boiling point includes all values
and subvalues therebetween, especially including 40, 50, 60, 70, 80, 90,
100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230,
240, 250, 260, 270, 280 and 290° C.

[0040]In the process according to the invention, the solvents used for the
carbamates may be alcohols, for example methanol, ethanol, isopropanol or
cyclohexanol, aliphatic hydrocarbons, for example hexane, heptane,
octane, nonane or decane, cycloaliphatic hydrocarbons, for example
cyclohexane or aromatic hydrocarbons, for example toluene or xylene. The
concentration of carbamate in the carbamate solution or else carbamate
suspension used is preferably 5 to 80% by weight, preferably from 10 to
50% by weight. The concentration of carbamate includes all values and
subvalues therebetween, especially including 10, 20, 30, 40, 50, 60 and
70% by weight.

[0041]It is advantageous in the process according to the invention to
initially charge the acid in the reaction zone at the desired reaction
temperature and only then to supply the carbamate to the reaction zone.

[0042]Preference is given to using 0.1 to 1000 molar eq., preferably 0.5
to 500 molar eq., of acid in the process according to the invention,
based on the amount of carbamate used. The amount of acid includes all
values and subvalues therebetween, especially including 0.5, 1, 5, 10,
50, 100, 200, 300, 400, 500, 600, 700, 800 and 900 molar eq., based on
the amount of carbamate used.

[0043]As a result of the use of acids of the formula II, a correspondingly
high reaction temperature is possible in the metered addition of the
carbamate during the cleaving in the process according to the invention,
the reaction temperature being sufficiently high for the cleaving of the
carbamate but also sufficiently low that transfer of the amine target
product to the gas and/or vapour phase can be suppressed. In this way,
the solvent which is introduced into the reaction zone through the
addition of carbamate can be removed again rapidly from the reaction
zone. The cleaving of the carbamate in the process according to the
invention is performed preferably at a reaction temperature of 50°
C. to 320° C., preferably of 120° C. to 250° C. The
reaction temperature includes all values and subvalues therebetween,
especially including 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160,
170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300 and
310° C. The reaction temperature is preferably selected such that
no amine target product can also be distilled over or be entrained with
the removal of the carbon dioxide. In general, an advantageous reaction
temperature in the bottom is one which is 20 to 30 K below the boiling
point of the amine.

[0044]The pressure at which the cleaving of the carbamate in the process
according to the invention is performed is preferably 20 mbar to 2000
mbar, preferentially 800 to 1200 mbar and more preferably 950 to 1100
mbar. The pressure includes all values and subvalues therebetween,
especially including 50, 100, 200, 300, 400, 500, 600, 700, 800, 900,
1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800 and 1900 mbar. Very
particular preference is given to performing the cleaving of the
carbamate in the process according to the invention at atmospheric
pressure.

[0045]The process according to the invention can be operated either as a
batchwise process or as a continuous process, but is preferably operated
as a continuous process.

[0046]In a preferred embodiment of the process according to the invention,
continuous delay reactors, for example tubular reactors, falling-film or
trickle-film evaporators, thin-layer evaporators or thin-film extruders,
are used. In these reactor types, in the process according to the
invention, the acid of the formula II is preferably circulated and the
carbamate to be cleaved--for example in the form of a solution or
suspension--is fed to the reaction zone in this apparatus. It is also
possible not to circulate the acid and thus to feed the acid back to the
reactor only after a separate workup or to discard it.

[0047]The cleaving forms carbon dioxide which can thus be removed
continuously from the reaction zone together with the solvent introduced
into the reaction zone--for example water. Owing to the high evolution of
CO2 during the cleaving of the carbamate, it is advantageous to
provide a large evaporator surface area; falling-film, trickle-film or
thin-layer evaporators are therefore particularly suitable for the
process according to the invention.

[0048]In order that the amine obtained after the cleaving of the carbamate
does not come into contact with the carbon dioxide which likewise forms,
thus resulting in a back-reaction to give the carbamate, the carbon
dioxide formed is stripped out by an inert gas stream, for example
nitrogen.

[0049]After the cleaving of the carbamate, the amine obtained from the
cleaving can be drawn off in the process according to the invention. This
is achieved, for example, by a distillative route by a further increase
in the temperature and/or lowering of the pressure, which removes the
amine from the reaction zone.

[0050]The remaining bottom phase comprising the acid of the formula II
may, in the process according to the invention, subsequently be provided
again to a cleaving of carbamates, by virtue of a downstream repeated
circulation or, in the simplest case, a further downstream semibatchwise
cycle, for example in a stirred tank reactor.

[0051]The amine obtained in the process according to the invention
generally has a sufficiently high purity that no further purification
steps are required. In particular, it is possible by means of the process
according to the invention to obtain amines having a GC purity of >98
area%. The purity includes all values and subvalues therebetween,
especially including 98.5, 99, 99.5, 99.8, 99.9, 100 area %. Should the
amine, however, not be in the desired purity, a further distillation or
rectification may follow.

[0052]It is advantageous to work under inert gas in all workup steps in
the process according to the invention in order to prevent the
reformation of carbamates which can reform from the amine reaction
product and CO2 from the atmosphere.

[0053]The examples which follow are intended to illustrate the process
according to the invention for preparing amines in detail without any
intention that the invention be restricted to this embodiment.

EXAMPLES

Example 1 (According to Invention)

[0054]A 11 flask with a stirrer and a 10 cm column with random packing, a
column head and a condenser was initially charged with 200 g of
neodecanoic acid (Versatic 10 from Resolution). This liquid phase was
heated to approx. 120° C. Within 2 hours, 800 g of a 30% by weight
aqueous solution of a carbamate of the formula I where R1=H and
R2═--(CH2)6--NH2 (this corresponds to 0.87 mol of
carbamate) were metered in. Within these 2 hours, approx. 550 g of water
were removed by distillation and approx. 19 1 of carbon dioxide were
released. The determination of the carbon dioxide formed was monitored
with a suitable gas meter. Once the carbon dioxide formation had ended
and all of the water had been removed, the pressure was reduced to such
an extent that the residual water can also be removed. With further
reduction of the pressure, 1,6-diaminohexane was then distilled off.

[0055]Approx. 95 g of 99% 1,6-diaminohexane were obtained; this
corresponds to a yield of approx. 95%.

Comparative Example 1

[0056]Comparative Example 1 was performed under conditions analogous to
those in Example 1, except that 200 g of Marlotherm SH were used instead
of 200 g of neodecanoic acid. No thermal cleaving of the carbamate was
observed. Merely distillative removal of the water was observed. The
uncleaved carbamate remained in the liquid phase.

Comparative Example 2

[0057]Comparative Example 2 was performed under conditions analogous to
those in Comparative Example 1, except that the bottom temperature was
heated to 200° C. Now, thermal carbamate cleaving was observed,
but the amine and carbon dioxide formed were present simultaneously in
the vapour space of the reactor, such that a back-reaction of the
carbamate was observed.

[0058]In Comparative Examples 1 and 2, no amine was isolated.

Example 2 (According to Invention)

[0059]A glass thin-film evaporator with a surface area of 0.1 m2 was
charged with approx. 500 g/h of neodecanoic acid with the aid of a
metering pump. The heat carrier feed temperature at the thin-film
evaporator was approx. 180° C. With the aid of a second metering
pump, the thin-film evaporator was charged with approx. 200 g/h of a 30%
by weight aqueous solution of a carbamate of the formula I where
R1═H and R2═--(CH2)6--NH2. Approx. 120
g/h of water were distilled off via the top. At the same time, as a
result of the C02 elimination, it was possible to monitor gas
evolution with the aid of a customary gas volume meter (approx. 4-5 l/h
of carbon dioxide were formed). Neodecanoic acid and 1,6-diaminohexane
remain in the bottom of the thin-film evaporator, which can subsequently
be separated from one another by distillation either on a further
thin-film evaporator or by simple short-path distillation.

[0060]The two process variants in Example 2 led to a yield of
1,6-diaminohexane based on the carbamate used of approx. 95%.

[0062]Numerous modifications and variations on the present invention are
possible in light of the above teachings. It is therefore to be
understood that within the scope of the appended claims, the invention
may be practiced otherwise than as specifically described herein.